Biophysicists develop new membrane channels

Carbon nanotubes nonspecifically transport molecules into cells

COURTESY OF WIKIMEDIA COMMONS  CARBON NANOTUBES, like the one pictured above, have been inserted into plasma membranes, where they can function as channels across which molecules and ions can diffuse. Future research will focus on controlling the flow through the nanotube.

COURTESY OF WIKIMEDIA COMMONS
CARBON NANOTUBES, like the one pictured above, have been inserted into plasma membranes, where they can function as channels across which molecules and ions can diffuse. Future research will focus on controlling the flow through the nanotube.

Matthew Reynolds
Science & Tech Editor

Nearly every biology course covers proteins and their functions, and membrane channel proteins comprise one of the most important classes. Membrane channel proteins are folded amino acids that span the lipid bilayers within and surrounding cells. All membrane channel proteins have a pore known as a membrane channel that specifically controls the relative concentrations of molecules and ions within the cell. This regulation of chemical concentrations among cells and their environments allows life processes to continue. The flow of chemicals through protein-based membrane channels can be regulated through altered gene transcription or “gating” of the channel by blocking the sites of entry for target molecules or ions. While the scientific community has known of these methods of channel regulation for decades, bioengineers have recently led investigations to see if they can manufacture a tube analogous to membrane channels.

A team of researchers led by Jia Geng, Ph.D., and Kyunghoon Kim, Ph.D., made strides in the field of biotechnology when the researchers recently published an article in the journal Nature about the insertion and specificity of carbon nanotubes (CNTs, also known as CNT porins). Materials scientists have known that one of carbon’s allotropes is a flat sheet of repeating hexagons, as in graphite, and materials scientists have been able to fold these sheets into carbon nanotubes. In fact, the width and structure of CNTs resembles that of the beta-barrel structural motif commonly found in protein-based membrane channels. Geng and Kim’s research team found that when 10-nanometer-long CNTs touch plasma membranes they spontaneously insert themselves, forming pores in the membranes much like the beta-barrel motif’s association with the membrane.

From this premise, the research team conducted experiments to see if CNTs have a high degree of specificity, like protein channels. To test water’s ability to traverse a lipid bilayer by means of CNTs, the team inserted spherical lipid vessels, known as liposomes that were filled with water into dilute solutions of sodium chloride and potassium sulfate. The results indicated a minimal change in size of less than 2.3 percent in control liposomes, while liposomes with CNTs shrank by up to 20 percent.

The team conducted further experiments, and the researchers found that when CNTs were present in a membrane surrounded by electrolyte-rich solutions, ions would flow across the membrane when different voltages were applied to the membrane. Meanwhile, the control membranes lacking CNTs displayed negligible ion flow across the membrane. Therefore, the team concluded that specific ions can traverse the CNTs as well as water.

Future bioengineering developments concerning CNTs undoubtedly will focus on controlling the specificity of the pore to mimic protein membrane channels more accurately and possibly increase effectiveness.

The team concluded with a hopeful message for further developments by stating in their Nature publication, “We expect that our CNT porins could be modified with synthetic ‘gates’ to dramatically alter their selectivity, opening up possibilities for their use in synthetic cells, drug delivery, and biosensing.”

 

Nov. 6, 2014

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